The presence of dissolved gases, even in small amounts, is undesirable in systems having aqueous solutions which contact metal surfaces. Metal surfaces in contact with oxygen-containing aqueous solutions can experience pitting. Pitting is highly concentrated corrosion affecting only a small area of the total metal surfaces. Pitting can be a serious problem causing metal failure even though only a small amount of metal is lost and the overall corrosion rate is relatively low.
The severity of attack by oxygen will depend on the concentration of dissolved oxygen in the aqueous solution, pH and temperature. As aqueous solution temperature increases, as for example in a water heating system, enough driving force is added to the corrosion reaction that small amounts of dissolved oxygen in the aqueous solution can cause serious problems. Oxygen pitting is considered to be a very serious problem in boiler systems, even where only trace amounts of oxygen are present.
Deaeration is a widely used method for removing oxygen from an oxygen-containing aqueous medium. It is particularly useful for treating boiler feedwater and can be either mechanical or chemical.
Vacuum deaeration has proven to be a useful mechanical deaeration method for treating water distributing systems. For example, boiler feedwater is commonly treated using pressure deaeration with steam as the purge gas. According to the pressure deaeration method for preparing boiler feedwater, the water is sprayed into a steam atmosphere and is heated to a temperature at which the solubility of oxygen in the water is low. About 95 to 99 percent of the oxygen in the feedwater is released to the steam and is purged from the system by venting.
Mechanical deaeration is considered an important first step in removing dissolved oxygen from water such as boiler feedwater. However, as already noted, as water temperature increases, even trace amounts of dissolved oxygen can cause serious problems. Accordingly, supplemental chemical deaeration is often required.
Oxygen pitting is also a problem in aqueous heat exchanger systems such as boiler feedwater heaters, economizers and superheaters.
Traditional chemical oxygen scavengers include sodium sulfite and hydrazine. However, sodium sulfite cannot be safely utilized in boiler systems operating at above 1000-1500 psi as corrosive hydrogen sulfide and sulfur dioxide can be formed at pressures above this range. Also, at these pressures, dissolved solids from the sulfite-oxygen reaction product can become a significant problem. Hydrazine is a toxic substance and is thought to be carcinogenic. Hence, its use is often undesirable.
It has been common in the past to combine an oxygen scavenger for treating boiler feedwater with a neutralizing amine for treating the steam condensate system of the boiler. Neutralizing amines are typically used in boiler systems to neutralize carbon dioxide in the condensed steam of the condensate system. This carbon dioxide is produced from the presence of carbonate or bicarbonate alkalinity in the boiler feedwater. Most deaerated boiler feedwater will contain alkalinity that undergoes decomposition at the higher temperature boiler operating conditions, releasing carbon dioxide with the steam. The most commonly used neutralizing amines for combining with oxygen scavenger have been morpholine and cyclohexylamine.
U.S. Pat. No. 4,192,844 discloses the use of methoxypropylamine neutralizing agent in combination with hydrazine to prevent corrosion in steam condensate systems.
Hydroquinone and hydroquinone mu-amine compositions have been used successfully as oxygen scavengers as described by Muccitelli in U.S. Pat. Nos. 4,279,767, 4,289,645, 4,487,708, 4,541,932, 4,549,968, 4,569,783, by Cuiba and Muccitelli in U.S. Pat. No. 5,147,604, by Cuiba in U.S. Pat. No. 4,282,111, and by Kerst in U.S. Pat. No. 4,278,635.
Rossi et al. in U.S. Pat. No. 5,256,311 discloses hydroxyalkylhydroxylamine oxygen scavengers for aqueous mediums.
Aminophenol compounds have been disclosed as oxygen scavengers in aqueous mediums in U.S. Pat. No. 4,728,497 to Muccitelli.
Feldman in U.S. Pat. Nos. 4,693,866 and 4,657,740 discloses the use of linear water soluble polyethyleneamines such as triethylenetetramine, tetraethylenepentamine, and pentaethylhexamines as oxygen scavengers when added to an aqueous medium. However, because of the variety of conditions in which oxygen induced corrosion of metal is a problem, a continual need exists for new oxygen scavengers.
Thus, it is an object of this invention to provide compositions and methods of using the compositions to scavenge oxygen from an aqueous solution.